Semiconductor materials are widely used for making electronic devices. The applications of semiconductor materials are directly related to our daily lives and in technology. Semiconductor materials have evolved from silicon or germanium (for example) in the first generation to more recent third generation materials such as silicon carbide (SiC) and gallium nitride (GaN). All electronic devices based on semiconductors are built on semiconductor substrates, and the pricing of electronic devices is a function of the cost of substrates. Generally, the first and second generation semiconductor substrates are grown from their respective melt, while the third generation semiconductor substrates are mostly grown from vapors, such as chemical vapor deposition (CVD), metalorganic chemical vapor deposition (MOCVD), physical vapor transport (PVT), hydride vapor phase epitaxy (HVPE), and such. The substrate costs of third generation semiconductor are much higher than in previous generations. As such, reducing the cost of substrates will benefit consumers of electronic devices, and prompt more applications, especially those based on the 3rd generation semiconductors.
Ion cut (i.e. smart-cut) technique was developed for producing silicon-on-insulator (SOI) wafer substrates in the 1990s. The ion cut actually includes ion cut and film transfer, using ion implantation to separate a film layer from its main substrate. The SOI fabrication process includes first forming a silicon dioxide film with thickness in the order of a few micro-meters on the donor silicon substrate. Next, hydrogen ions are implanted into the silicon substrate through the silicon dioxide to reach the silicon substrate and stays in the silicon substrate, which cause an ion damage layer in silicon. The depth of ion damage layer under surface is determined by the ion acceleration voltage. That is, the higher the acceleration volts, the deeper the ions can travel into the substrate. The depth of the ion damage layer under the surface of the silicon substrate usually ranges from a micrometer to twenty some micrometers. A bonding process follows the ion implantation step, in which an acceptor silicon substrate is bonded to the donor silicon substrate on the silicon dioxide surface. Bonded substrates are then annealed at temperatures of 200-500° C. The hydrogen ions in ion damage layer agglomerate into hydrogen gas during the annealing. The hydrogen gas extends along the ion damage layer and causes the separation of the donor substrate from acceptor substrate at the ion damage layer. The donor wafer can then be reused. The acceptor wafer undergoes further heat annealing to rearrange the silicon atoms on the separate surface. The surface atom rearrangement eliminates the ion damage. A silicon film of a few micrometers in thickness lies above the silicon dioxide layer. Electronic devices can then be built on this silicon film. The acceptor substrate only supports the silicon film and silicon dioxide layers above it. The acceptor substrate, with Si film and SiO2 layer on it, is the SOI substrate. The ion cut technique has also been attempted on other semiconductor materials to transfer film. To reduce substrate cost for SiC and GaN, great efforts were made to transfer SiC and GaN film to Si or oxide substrates by means of ion cut. In contrast from the SOI process, there is no requirement to form an oxide layer on GaN or SiC, because they are usually transferred on insulators. In the ion cut process, there are some important factors such as implanted ion dosage, depth of implanted ions, bonding of acceptor substrate to donor substrate, and annealing after bonding. Among these factors, bonding of the acceptor substrate to the donor substrate is critical to the success of ion cut and film transfer. If donor and acceptor wafers are not bonded very well, the film to be transferred can be broken during annealing or separation due to lack of support from the acceptor wafer. To achieve satisfactory bonding, the bonding surface of substrates must be very flat. Getting the surface of SiC substrate to meet the bonding requirements is very challenging because of the hardness of SiC. In addition, the separated surface in a normal ion cut process needs a further process like heat annealing or polishing to repair damage to the surface and to allow electronic devices to be formed thereon.
In last a few years Stephen W. Bedell, et. al., successfully separated Si, Ge, GaAs, and GaN films from their respective substrates by means of a controlled spalling. The process of controlled spalling includes first coating a stress inducing layer on top of a semiconductor substrate first, and then putting a tape on the stress inducing layer. By pulling the tape, a film of substrate attached to the stress inducing layer is separated from the main body of the substrate. The thickness of the film that is separated from the substrate is controlled by stress in the stress inducing layer. This thickness control method may cause difficulty in large scale production. Furthermore, there is no reported application of controlled spalling on hard substrate materials such as SiC although it can be done theoretically.